Experimental measurement of surface profiles, such as surface-wave elevation profiles, is important for many free-surface flow studies. With the increasing use of computational tools in connection with complex nonlinear wave dynamics around surface vessels in deep-ocean and coastal regions, time-resolved three-dimensional surface-wave data is important in model development and in the validation of flow solutions. For instance, wave-induced motions of a surface ship in severe weather can cause passenger discomfort, large hull loads, cargo damage, and even can cause the ship to capsize. Current ship motion prediction programs typically consider various components of external forces, including hydrostatic restoring and Froude-Krylov wave forces, radiation and diffraction forces, viscous roll damping, and appendage forces from propellers, bilge keels, rudders and fins. Usually, each of the components of forces is computed separately and linearly superposed to yield total forces and moments. In cases where nonlinear wave interactions are significant, it is necessary to develop more sophisticated models and to validate predictions using experimental data on the wave field around the ship and associated six-degree-of-freedom ship motions. The problem becomes even more complex with the consideration of multi-hull designs where wave-making interaction of various waterplane sections can play a significant role in resistance and seakeeping.
In model testing, various measurement techniques, such as techniques using sonic and electromechanical devices, are used to measure surface wave elevation at a small number of discrete locations. One example of such a technique is the “whisker probe,” which uses a stainless steel needle or whisker mounted on a support arm to probe the surface of the water by electrical conductivity. Upon entry of the whisker into the water, the small electrical potential on the whisker is grounded out, and a positional readout proportional to the water level is output. The probe then begins retracting the whisker out of the water until the meniscus on the needle tip breaks, at which point the whisker is again lowered into the water to begin another cycle. By keeping the whisker arm very light, data rates of up to 60 measurement cycles per second have been achieved, although operation in the 30 to 40 Hz range is more practical.
The use of electromechanical devices, such as the whisker probe, has been mostly limited to the measurement of steady wave profiles for at least two reasons. First, the mechanical design of the whisker probe does not allow the measurement of large peak-to-peak waves. Second, it is impractical to employ more than a few electro-mechanical devices simultaneously in an experiment. For many seakeeping and maneuvering problems, particularly those involving large-amplitude waves and ship motions, it is desirable to obtain dynamic measurement of the three-dimensional wave surface at a large array of points around a ship model.
Currently, there are a limited number of techniques that allow the measurement of complex three-dimensional surface of water waves. These include the measurement of surface slope by intensity-based reflection, color-encoded optical reflection, and light shadowgraphy. These techniques, however, are limited to the measurement of small wave slope over a small physical area and are not suitable for practical large-scale surface wave measurements in tow-tank facilities.